Which of the following is a client-server program that opens a secure, encrypted command-line shell session from the Internet for remote logon?

Secure Shell (SSH) is a cryptographic network protocol designed for secure remote login and other secure network services over an unsecured network. It operates on a client-server model where an SSH client application connects to an SSH server. The protocol provides a secure, encrypted channel for a command-line shell session, ensuring the confidentiality and integrity of all data transmitted, including passwords and commands. This directly matches the description in the question.

A. A VPN (Virtual Private Network) creates a secure network tunnel but is not itself a command-line shell program for remote logon.
B. IPsec (Internet Protocol Security) is a protocol suite that secures network communications at the IP layer, often used for VPNs, but it is not a user-facing remote logon application.
D. SFTP (SSH File Transfer Protocol) is a protocol for secure file transfer that runs over an SSH session; its primary function is transferring files, not providing a general command-line shell.

1. Ylonen, T., & Lonvick, C. (2006). The Secure Shell (SSH) Protocol Architecture. RFC 4251, IETF. Section 1, Introduction. "The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network."

2. Saltzer, J. H., & Kaashoek, M. F. (2009). Principles of Computer System Design: An Introduction. MIT OpenCourseWare, Chapter 9, Security. The chapter describes SSH as the primary mechanism for secure, encrypted command-line access to remote systems.